There are presently in existence in the United States approximately 190,000 emergency locating transmitters (ELTs) required by law on all small aircraft travelling more than twenty-five miles from an airport and about 6,000 emergency position indicating radio beacons (EPIRBs) required on certain classes of marine vessels. ELTs and EPIRBs are essentially the same device which transmits an audio tone on 121.5 MHz and 243 MHz indicating that a distress incident has occurred. In general, the ELTs are activated by gravitational forces (impact) whereas the EPIRBs are activated in response to water. The audio tone generated by these devices is provided by a distress modulation waveform having a downward sweep of at least 700 Hz within a band of 1,600 Hz and 300 Hz at a rate of 2 Hz to 4 Hz. These specifications are predetermined by legislation so that the required transmitted signals may be recognized by individuals listening for the characteristic tone on the 121.5 MHz and 243 MHz carrier frequencies. The Air Force Rescue Coordination Center (AFRCC), for example, receives information from others, i.e., FAA flight service stations, airports, Civil Air Patrol, etc, who monitor the 121.5 MHz and 243 MHz carrier frequencies and respond to distress signals by notifying search and rescue personnel (SAR), such as the Civil Air Patrol, to search for the location of the distress transmission and initiate rescue operations. Since the distress transmission contains no information other than the fact of a distress, however, SAR have no way of obtaining any advance information concerning classification of the distress. It would be very helpful to SAR to know, for example, whether the source of the distress signal is a downed aircraft, marine vessel, camper, hiker or ski patrol to better coordinate rescue operations. There is presently no means for determining whether the distress signal is associated with any of these classes of users or even whether the signal is generated by a civilian or military craft (whose ELTs transmit only 243 MHz) using an ELT or an EPIRB. There exists a need, therefore, for some means to enable determination of classification of the user initiating a distress transmission.
Since the characteristic distress signal generated by ELTs and EPIRBs comprise carriers that are modulated by the characteristic distress tone, which demodulates to a siren like sound, the distress signal is difficult to detect electronically. Although the signal has been developed primarily for detection by a human operator, there is presently a need for electronic equipment that may be carried in satellites to augment existing search and rescue capabilities. Since specifications for the distress signals transmitted by ELTs and EPIRBs have been determined by legislation, however, it is not possible to significantly depart from the present specifications.
One object of the present invention, therefore, is to provide a method of and apparatus for identifying user classification of an ELT or EPIRB generated distress signal.
Another object is to provide a method of and system for identifying user classification in an ELT or EPIRB generated distress signal without departing from the legislated range of signal specifications.
Another object is to provide a method of and system for identifying user classification in an ELT or EPIRB generated distress signal without requiring complex transmission circuitry and wherein existing ELT and EPIRB units can be adapted to generate a user classification identification signal.